Metal drawing procedure



United States Patent 3,256,731 METAL DRAWING PROCEDURE Erle V. Peterson, Linthicum Heights, Md., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Aug. 30, 1965, Ser. No. 483,853 1 Claim. (Cl. 72-377) This invention pertains to processes for preparing elongated metal sections for drawing through dies to impart to such sections their final shape within specified dimensional tolerances. More particularly the invention pertains to preparing the forward end of such a section to facilitate feeding the section into a draw die, by forming on said end a lead point long enough to protrude through said draw die and thence into the jaws of a gripping device for drawing the section through said die, and comprises the improvement of (1) dissolving off metal throughout the length of said point until the point has an average thickness slightly more than the corrseponding openings of a selected sizing die and yet is thin enough to permit said point to be pushed with force into said sizing die, (2) pushing said point substantially to its full length into said sizing die, and (3) removing said point from said sizing die, said sizing die having thickness openings in the range from .001 to .005 inch less than the corresponding openings in said draw die. I

The elongated metal sections to which this invention applies are usually prepared by extruding an ingot through a die to produce an extrusion having the approximate final shape but slightly larger to allow for additional reduction to be obtained by pulling, i.e., drawing, the extrusion through another similarly shaped finishing die or a series of them. This drawing produces straighter products having better finish and greater dimensional accuracy than is usually possible to achieve by extrusion, especially in the case of high melting or refractory metals. The extrusions for drawing are usually relatively long and thin and may be prepared in any desired cross-sectional shape, such as round and angular rods and tubes, H section, V section, T section, I sections, star shapes, finned tubes, etc. The more complicated the cross-section, the more difficult it is to obtain close tolerance and good finish in the product.

This invention is limited essentially to preparation of the work piece for the first step in the drawing operation-namely, that of inserting the forward end of the extrusion into the draw die. Since the end of the extrusion is necessarily larger than the opening in the draw die some provision must be made for getting enough of the forward end through the die to permit gripping of such end with the drawing clamp. Once this positioning is accomplished, the whole work piece is pulled through the die, giving the piece a minor degree of overall reduction in thickness while preserving the same general shape and bringing it nearer to the final desired specifications of cross-sectional dimensions and degree of surface perfection. As a rule the work piece becomes thinner in all its parts and corresponding longer while keeping the same overall size. While tubing may be reduced somewhat in diameter the drawing operation may be directed chiefly toward making the wall thinner and more uniform.

The amount of metal reductionthat is, the decrease in cross-sectional area of the solid metalwill vary considerably. Usually, it is practical to get as much reduction as possible per pass through the die. The maximum possible reduction will depend on the ability of the work piece itself to withstand the forces applied to pull it through the die. Lubricants of course may be applied to reduce these forces, but nevertheless the drawing operation poses the problem of balancing the strength of the work piece against the drawing force used.

The forward end of the work piece must be made smaller than the draw die for a distance of from 6 to 10 inches in order to begin the drawing. It is well known that this smaller lead, or point, section is the weakest part of the work piece and will determine the maximum degree of reduction possible per pass in most cases. One must therefore strive to keep the maximum amount of metal in it which will slip through the die under whatever force can be applied by pushing the point into the die. For convenient operation an easy fit is desired. The process therefore requires that enough metal be taken from the point to permit easy insertion but no more. In the past this has been accomplished by pickling the end in acid to reduce the thickness nearly to the desired point and then filling or grinding off the remainder thereby removing the uneven etch pits and particularly the inside corners, etc., left by the acid. This is time-consuming and expensive.

. The improvement embodied in this invention comprises a carefully controlled combination of pickling, that is, dissolving off metal from the lead end of the work piece by chemical action, followed by pushing the pickled end into a sizing die selected to conform to the cross-section of the work piece, and slightly smaller, i.e., in the range of l to 5 mils, than the ensuing draw die. The minimum amount of metal removed by the pickle is determined by the ability of the metal work piece to withstand the force necessary to push it through the sizing die. The maximum amount is substantially no more than that which will allow the pickled end to fill the sizing die throughout the length of the point. The removal of any more metal weakens the point severely, since the point is usually strengthened by even the small amount of cold work imparted by' passing the point into the sizing die. It is preferable to remove a minimum of metal in the pickling solution, which may be a mineral acid such as sulfuric, hydrochloride, or nitric, an acid salt, etc. This minimum may be lowered by pushing the point into the sizing die in steps which can be done in a jig which grips the point near the end for each of a succession of short pushes until the entire length of the point has entered the sizing die. After this sizing of the point, the work piece is inserted easily in the draw die and drawn as usual.

Reference herein to a point or lead point will be understood to mean a lead section of diminished thickness or diameter. Thus, in the case of a rod, the point might be a forward section in the shape of a right cylinder of smaller diameter than the rest of the work piece. There is no implication that the point is, in this instance, coneshaped, and the same is true for other shapes. By pushpointing is meant the forming of a lead section of diminished thickness by pushing the lead section into a sizing die of smaller dimension.

In speaking of the thickness of the work piece and the corresponding die openings, reference is made in the case of tubing to the wall thickness, in the case of complex parts to the web and flange thickness, etc. It will be understood that in the drawing operation the die design adheres to the essential requirements and specifications applicable to the metal drawing art, particularly with respect to the degree of reduction in thickness obtained per pass when different members of the work piece have different thicknesses,

The processes of this invention can be applied particularly advantageously to the forming of thin sections of refractory metals, where neither push-pointing nor pickling alone of the lead point will sutfice. While pushpointing alone might in some cases reduce the point to the desired size, this cannot be accomplished by a single pass. For example, if a contemplated drawing operation requires a 10% reduction in thickness of the section the point must be reduced somewhat more, say 12%. This is almost always impossible because the metal is not stiff enough to withstand even the same force for pushing through a die that it can survive under the tension of the drawing operation. Push-pointing alone would then re quire a succession of pushes through a series of finely graded dies to obtain the desired 12% reduction. This becomes impractical in terms of time and die cost.

Pickling alone is clearly impractical and frequently inoperative because of its non-uniform action. If pickling is continued until enough metal is removed to permit insertion in the draw die, localized thin places are developed which initiate failure when drawing is attempted.

The combination of pickling and single push-pointing practiced by this invention shows distinct advantages resulting from the co-operative effect of the two steps. For example, when the cross-section of the work piece is angular, as in an H section, the web and flanges may generally be pickled almost to size, but the inside corners are less rapidly attacked, leaving enough metal to interfere with insertion in the draw die. Pushing the point through the sizing die opens these corners satisfactorily, providing a close fit in the draw die and a point having maximum strength. The combination of steps accomplishes the result, which is impossible by pickling alone, efficiently and at relatively low cost.

The invention will be better understood by reference to the following illustrative example:

Example A billet of a columbium base alloy (D-43) was extruded to have a cross-section in the shape of an H nominally one inch wide and 1% inches high with webs and flanges .07 inch thick 1.002 inch. It was desired to draw this piece, which was about 80 inches long, to a piece having a cross-section of the same width and height but having a thickness of .040 inch. After extrusion, the whole piece was pickled mildly and sanded to clean the surfaces. From experience it was known that this amount of reduction would require three passes through draw dies of successively smaller openings. To prepare the point for insertion in the draw die, the back end of the extrusion was immersed to a depth of 10 inches in an agitated acid bath comprising 40 parts by volume of 63% nitric acid and 60 parts by volume of 48% hydrofluoric acid and 100 parts by volume of water at 25 C. Periodically, the work piece was removed from the acid, rinsed and micrometered at the center of both the web and flanges at locations 1, 5 and 9 inches from the end. The pickling was continued until these measurements averaged .0535 inch. The piece was then mounted in a jig adapted to push the pickled end through a sizing die held in proper alignment by the jig. Hydraulic means for foreing the point into the die for a distance of 10 inches and withdrawing it were provided. The die was contoured and had web and flange openings corresponding to a thickness of .053 inch. The point was lubricated by mer, and was then clamped in the jig and pushed into the sizing die. After push-pointing in the manner indicated, the point was smooth and was inserted into the draw die having flange and web openings corresponding to .056 inch. The drawing was done in the 400-700 F. temperature range using the same lubricant. Temperature was found not to be critical within this range. The 10- inch point was again pickled as before to prepare it for pushing into a .042 inch sizing die for drawing in an .045 inch draw die. The pickled point averaged .044 inch in thickness and variations from .039 to .051 inch were found. This point, being thinner than the previous one and being slightly larger relative to the sizing die was deemed not stiff enough to stand the full 10-inch push in one step without bending or collapsing. Consequently, it was clamped by the web in the jig two inches from the end and pushed. Thus, in five successive pushes the 10- inch point was sized for drawing the whole section in the next draw die to a thickness of .045 inch. This process was again repeated, pickling to about .036 inch, pointing in a .035 inch sizing die and drawing in a .038 inch draw die. The finished section was of good quality, and the thickness of web and flanges held within .038 to .0405 inch.

The method shown in the example proved much more satisfactory than the previous procedure of pickling until the point could be inserted in the draw die, or pickling combined with filing or grinding. In either of the latter cases the result was either very time-consuming or the points were weak so that actual parting occurred during drawing unless more numerous, expensive draw dies were provided. Breaking of the points usually occurred at the start of the draw so that the process had to be repeated from the beginning.

I claim:

In a process for preparing an elongated metal section for a drawing operation in which there is first formed on the section a lead point long enough to protrude through a draw die and thence into the jaws of a gripping device for drawing the section through said die, the improvement which comprises the steps of (1) dissolving off metal throughout the length of said point until the point has an average thickness of slightly more than the corresponding openings of a selected sizing die and yet is thin enough to permit said point to be pushed with force into said sizing die, (2) pushing said point substantially to its full length into said sizing die, and (3) removing said point from said sizing die, said sizing die having thickness openings in the range from .001 to .005 inch less than the corresponding openings in said draw die.

References Cited by the Examiner UNITED STATES PATENTS 383,240 5/1888 Palmer 29-558 669,698 3/1901 Ivins 7242 2,268,525 12/ 1941 Roesner 72377 CHARLES w. LANHAM, Primary Examiner.

H. D. HOINKES, Assistant Examiner. 

